1. Field of the Invention
This invention relates to a charge storage tube and a method for operating the same, most suitable for a signal conversion type or an image pickup type charge storage tube wherein an electric signal or a photo signal is storaged and the storaged signal is read out as an electric signal.
2. Description of the Prior Art
The conventional charge storage tube 1 as shown on FIG. 1 includes a vacuum tube 2 made of glass, in which an electron gun 3 for writing a signal and an electron gun 4 for reading the signal are opposite to each other, and a target 5 is arranged between the electron guns 3 and 4. The electron gun 3 comprises a cathode 6, a control electrode 7 and an accelerating anode 8. The electron gun 4 also comprises a cathode 9, a control electrode 10 and an accelerating electrode 11. The cathode 6 is earthed through a power source 12 in common with the cathode 9 and connected to the electrode 7 through a power source 13 and a resistance 14. In the tube 2, focusing and deflecting electrodes (not shown) are disposed in order to scan electron beams 15 and 16 from the electron guns 3 and 4 on the given positions of the target 5 respectively. The target 5 is earthed through a resistance 17 and a power source 18. As shown on FIG. 2, the target 5 comprises an N-type semiconductor substrate 19 and an insulating layer 22 such as a SiO.sub.2 layer having openings 21. P-type semiconductive regions 20 are formed on the surface of the semiconductor substrate 19 at a regular interval by diffusing impurities through the openings 21 into the semiconductor substrate 19. An N.sup.+-type semiconductive region 23 is formed on the back surface of the semiconductor substrate 19, whereby pairs of electron-hole below mentioned are prevented from recombining.
Next, the operation of the charge storage tube 1 will be described.
First, the electron beam 16 from the cathode 9 is scanned uniformly on the surface of the target 5, whereby the potential of the surface of the P-type semiconductive regions 20 existing in the openings 21 become equal to that of the cathode 9. In this case, each PN junction diode formed by the P-type semiconductor regions 20 and the semiconductor substrate 19 is reversely biased or electrically charged by the formation of depletion layer, since a plus potential of about 10 V is supplied to the semiconductor substrate 19 from the power source 18. And then a signal voltage is supplied to the electrode 7 of the electron gun 3 to generate the electron beam 15, which attacks the given positions of the semiconductor substrate 19 when the potential of about -2 KV is given to the cathode 6 by the power source 12. Consequently, pairs of electron-hole are generated in the semiconductor substrate 19 by signal writing beams with an accelerating energy and a signal corresponding to the potential of the cathode 6. The pairs of electron-hole diffuse in non-electric field region within the semiconductor substrate 19. The holes drift in the depletion layer of the above-mentioned diode to neutralize negative charges stored in the P-type semiconductive region 20, whereby the charges are electrically discharged. As a result of the discharge of electricity, the surface potential of the P-type semiconductive regions 20 become higher and an image corresponding to the above-mentioned signal is formed on the diode so that the writing operation ends. Next, at this condition, the electron beams from the electron gun 4 are scanned uniformly against the target 5 in order to charge the discharged diodes electrically again. The charging currents are taken out as output signal whereby the reading operation ends.
According to the charge storage tube 1, however, a written signal is erased by the reading operation since the writing operation and the reading operation are done by the charge and the discharge of the PN junction diode as described above. Accordingly, the written signal is read out, as it were, by destroying it and therefore it is impossible to retain the written condition. Moreover, the written condition can not be retained even for more than one second due to dark currents or reverse currents passing through the PN junction, whereby the written signal disappears without doing the reading operation.